Optical devices which use liquid crystal displays, for example, optical shutters, light adjusting glasses, alphanumeric display panels, and the like, are well known in the art. Such devices use liquid crystals selected from among the many known types which, when subjected to an electric current or electromagnetic field, can be caused to change their orientation so that they are arranged in a light-scattering (opaque) mode or light-transmitting (transparent) mode. The devices are generally composite layered constructions having at least one liquid crystal layer sandwiched between two transparent substrates, or one transparent substrate and one reflecting substrate. The transparent substrates can be made of glass, synthetic polymer films, etc., and typically include a thin transparent electrode material layer or pattern on the surface in contact with the liquid crystal layer.
In the past, in order to manufacture liquid crystal displays, transparent substrates having transparent electrodes and oriented polymeric films on them were prepared. The transparent substrates, with the transparent electrodes and oriented films facing each other, were spaced apart with numerous spacers, several micrometers in diameter, interspersed between them to provide a constant gap. Liquid crystals were injected into the gap between the substrates to form a liquid crystal layer between the substrates. Such injection operations were extremely time consuming, requiring 6 to 12 hours to produce elements for displays having large image planes (about 10 inches, or 25.4 centimeters, in size).
More recently, liquid crystal display layers consisting of a porous matrix material in which liquid crystal compounds are contained has been proposed. For example, Japanese Laid-Open Patent Application 56-11436 discloses an article consisting of a porous matrix sheet of acrylonitrile-based copolymer, polypropylene, or polycarbonate impregnated with liquid crystals, laminated between two sheets of 50 micrometers thick polyester film having transparent vapor-deposited indium oxide electrode layers on them. Although improving the manufacture of liquid crystal displays, the article described above exhibits poor retentivity of the liquid crystals introduced into it, so the liquid crystals must be impregnated into it, by a time consuming and difficult procedure, at the, time the liquid crystal display is manufactured. Also, the electrode substrates, i.e., the polyester cover films, of the article require an orientation treatment in order to obtain desired orientational properties in the liquid crystals.
In Japanese Laid-Open Patent Application No. 1-198725 and its U.S. counterpart, U.S. Pat. No. 5,304,323 (to Arai, et al.), is disclosed a liquid crystal layer formed by first mixing together a low molecular weight liquid crystal compound and a polymerizable monomer miscible with the liquid crystal compound. The mixture is then coated onto a substrate and laminated to another substrate, or infused between two substrates, after which the monomer is polymerized to form a polymer matrix. During polymerization, phase separation of the forming polymer and the nematic liquid crystal takes place resulting in a continuous liquid crystal phase in a three-dimensional polymer matrix. These devices, however, suffer problems such as low electrical resistance and low durability resulting from contamination of the liquid crystal phase by polymerization catalysts or impurities in the monomer; or difficulty in developing desired strength properties in the matrix material.
In European Patent Application No. EP 0 512 397 A2 (to Yoshinaga, et al.) is disclosed a liquid crystal device in which the liquid crystal display layer comprises a porous polymeric matrix, preferably formed from a polymer film from which all contaminants have been removed. Preferably the polymer film is stretched to develop porosity which eliminates the likelihood of contamination from pore-forming materials. The porous matrix films are preferably of polymers having a surface energy of 20 dyne-cm or less; especially preferred is polytetrafluoroethylene. The porous polymer film, preferably about 50 micrometers thick, is laminated using an adhesive between substrates which may include transparent electrodes of indium-tin-oxide (ITO), SnO.sub.2, or a metal, on the surfaces to be contacted by the liquid crystals. A liquid crystal compound is then injected into the space between the substrates and drawn in by capillary action and/or vacuum to fill the pore volume of the matrix material which, as noted above, is a difficult, time consuming, and expensive procedure. To overcome the above-listed problems associated with assembling liquid crystal display devices, the inventors of the instant application prepared a precursor assembly for a liquid crystal display device as disclosed in European Patent Application No. EP 0 599 483 A1 (to Okino, et al.). The precursor assembly comprises a porous polytetrafluoroethylene film having its pores substantially filled with nematic liquid crystals. Both surfaces of the liquid crystal-filled polytetrafluoroethylene film are covered by non-porous synthetic polymer films and the edges of the layered assembly are hermetically sealed. The precursor assembly is easily and quickly manufactured and permits long term storage of liquid crystals in ready-to-use sheet form for easy inclusion into a liquid crystal display. In use in a liquid crystal display device, the precursor assembly provides highly contrasting bright (transparent) and dark (light scattering) modes, however, it suffers the drawback that the rates at which it switches from one mode to the other is slower than is desired.